d
$\frac{F}{A_{1}}=Y \frac{\triangle l_{1}}{\lim }$ plies $\triangle l-1=\frac{F l}{A_{1} Y}$
$\frac{F}{A_{2}}=Y \frac{\triangle l_{2}}{\lim }$ plies $\triangle l-2=\frac{F l}{A_{2} Y}=\frac{F l}{4 A_{1}}=8 m m$
$\Delta l-1+\triangle l_{2}=10 m m$
$\frac{F l}{A_{1} y}+\frac{F l}{4 A_{1} Y}=10 m m \Rightarrow \frac{F l}{A_{1} Y}=8 m m$
